Fluvial floodplains prior to greening of the continents : stratigraphic record, geodynamic setting, and modern analogues.

Abstract

Fluvial floodplains established prior to the greening of continents have long been overlooked, despite their relevance for landscape reconstructions in deep time. The record of fluvial overbank sedimentation dates back as far as theMesoarchean, and mature assemblages of floodplain landforms had already developed at least by the early Palaeoproterozoic. In this review, a critical assessment of pre-vegetation floodplain processes and deposits is carried out through literature compilation and detailed descriptions of case studies. Pre-vegetation floodplains were variably composed of floodbasins, splay complexes (including crevasse- and distributary-channel fills and related splay lobes) and, in minor proportion, by channel levees. The hydrology of ancient floodbasin environments, mainly inferred from the occurrence or lack of evaporite features, is particularly topical and, once critically tested against other palaeo-environmental indicators, can be related to climate or catchment physiography. Pre-Silurian floodplains preferentially developed in rift basins prone to restricted drainage, where low-gradient axial depressions experienced limited stream power and accumulation of cohesive fines. Since supercontinents prone to host mature intracratonic basins first appeared in the Palaeoproterozoic, a causal relationship is established between the rise of modern-style plate tectonics and fluvial floodplains. By comparison, prevegetation overbank records are sparse in foreland, syn-orogenic, or passive-margin basins, where higher gradients and ocean-ward bypass of cohesive fines would have enhanced reworking by adjacent channels. These features are analogue to modern non-vegetated floodplains, with examples drawn from both arid endorheic drainages (Death Valley, California; Salar de Uyuni, Bolivia), and humid exorheic drainages (coastal plains of southern Iceland). Future developments in pre-Silurian sedimentology will help addressing lingering questions related to, for instance, the biogeomorphology of microbial mats and the morphogenetic variability between floodplains developed in different latitudinal climate belts.